JP3347481B2 - Exhaust gas purification catalyst - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to carbon monoxide (C) contained in exhaust gas from an internal combustion engine such as an automobile engine.
O), a hydrocarbon (HC) and a nitrogen oxide (NOx).

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by simultaneously oxidizing CO and HC and reducing NOx has been used as an exhaust gas purifying catalyst for automobiles.
As such a three-way catalyst, for example, a catalyst-supporting layer made of γ-alumina is formed on a heat-resistant carrier made of cordierite or the like, and the catalyst-supporting layer supports a platinum group element such as platinum or rhodium. Is widely known. Also known is a three-way catalyst in which ceria (cerium oxide) having an oxygen storage effect is used in combination to enhance low-temperature activity.

[0003] In recent years, the exhaust gas purifying catalyst is installed at a position immediately below a manifold close to the engine, and the exhaust gas temperature becomes high during high-speed running, so that the exhaust gas purifying catalyst is exposed to high temperatures. In many cases. However, an exhaust gas purifying catalyst supporting a platinum group element or cerium oxide tends to deteriorate at a high temperature and has a problem that purification performance after durability is reduced. Therefore, it has been considered to improve the high-temperature heat resistance by further supporting a rare earth metal or alkaline earth metal oxide or by using palladium having good heat resistance as a platinum group element.

However, palladium is inferior in NOx purification performance in a rich atmosphere and slightly leaner than the stoichiometric air-fuel ratio (stoichiometric). Therefore, it is considered that purification of NOx is difficult only with palladium, and it is used as a three-way catalyst when used in combination with rhodium having high NOx purification performance. However, since rhodium is very expensive, it becomes an expensive exhaust gas purifying catalyst.

[0005] Therefore, only palladium is used as the platinum group element, and in addition to the above, the exhaust gas which has a carrier layer containing cerium oxide, barium compound, zirconium compound, lanthanum oxide, activated alumina, etc., is inexpensive and has excellent ternary activity. Purification catalysts have been proposed (Japanese Patent Laid-Open Nos. 5-168926 and 5-68927).

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
Three-way catalyst combining palladium with various co-catalysts
The water gas shift reaction (C
O + H_TwoO → CO_Two+ H _Two) And steam reforming reaction (HC +
H_TwoO → CO_Two+ H_Two) To improve NOx purification performance
Although it is raised, especially on the slightly lean side than stoichiometric
NOx purification performance is not sufficient.

The present invention has been made in view of the above circumstances, and an object of the present invention is to improve a three-way catalyst using palladium as a catalyst component to further improve NOx purification performance in a slightly leaner atmosphere than stoichiometric conditions. And

[0008]

The exhaust gas purifying catalyst of the present invention which solves the above-mentioned problems is provided by a three-way catalyst system.
Exhaust gas discharged from the engine for which the feedback control is performed
An exhaust gas purifying catalyst for purifying exhaust gas, comprising a carrier substrate, a catalyst supporting layer formed on the surface of the carrier substrate, and a catalyst comprising palladium, platinum and an alkaline earth metal supported on the catalyst supporting layer. And platinum is supported only on the upstream side with respect to the flow direction of the exhaust gas flow.

[0009]

In the exhaust gas purifying catalyst of the present invention, palladium, platinum and an alkaline earth metal are supported. With this catalyst, NOx in the exhaust gas discharged from the air-fuel mixture in each state from the rich atmosphere to the lean atmosphere is purified as follows. (1) Rich atmosphere side The alkaline earth metal as a promoter component promotes a water gas shift reaction and a steam reforming reaction, and high NOx purification performance is obtained. (2) Near stoichiometric air-fuel ratio Palladium excellent in ternary activity mainly purifies NOx. (3) Lean atmosphere side Platinum mainly purifies NOx.

By the way, platinum has a characteristic that the purification performance is excellent under the condition that the amplitude of the air-fuel ratio (A / F) is large. Therefore, in the present invention, platinum is carried on the upstream side of the exhaust gas flow. This improves the NOx purification performance near the stoichiometric air-fuel ratio and slightly on the lean side as compared with the conventional catalyst supporting palladium and alkaline earth metal. When the volume occupied by the platinum-carrying portion increases, the characteristics of platinum appear strongly and the characteristics of palladium excellent in ternary activity decrease, and the purification performance of NOx near the stoichiometric air-fuel ratio and slightly on the lean side decreases. Therefore, it is desirable that platinum is supported on the upstream side in the range of 15 to 50%, preferably 15 to 30% based on the whole volume of the carrier.

If the amount of supported platinum and palladium is configured so that palladium is contained in a larger amount than platinum, the characteristics of palladium excellent in ternary activity near the stoichiometric air-fuel ratio and the NOx purification performance in the lean atmosphere side are improved. The excellent balance of the characteristics of platinum is optimized, and the NOx purification performance near the stoichiometric air-fuel ratio and in a slightly leaner atmosphere is improved. In particular, the range of 1/50 ≦ Pt / Pd ≦ 1/2 is desirable.

[0012]

The present invention will be specifically described below with reference to examples. In the following examples, “parts” means “parts by weight” unless otherwise specified. Example 1 100 parts of activated alumina powder, 40 parts of cerium oxide powder stabilized with zirconium oxide, 30 parts of lanthanum carbonate powder, 65 parts of a 40 wt% aluminum nitrate aqueous solution and 80 parts of water were mixed to prepare a slurry for coating. did.

[0013] A cordierite honeycomb monolithic carrier substrate (volume 1.3 L, total length 143.5 mm) is immersed in water, excess water is blown off, then immersed in the slurry, taken out, and extracted. 250 ° C
For 1 hour, and calcined at 500 ° C. for 1 hour to form a catalyst supporting layer mainly composed of alumina. The coating amount of the catalyst supporting layer is 190 g per liter of the carrier substrate.

The entire carrier thus obtained was immersed in an aqueous solution of palladium nitrate having a predetermined concentration for 1 hour, pulled up to blow off excess water, and dried at 250 ° C. Next, the entire palladium-supported carrier is immersed in water, and excess water is blown off. Then, only the portion 65 mm from the upstream end surface is immersed in a predetermined concentration of dinitrodiammine platinum aqueous solution, and then immersed for 1 hour and pulled up for extra. 250
Dried at ° C.

Further, the entire carrier supporting palladium and platinum was immersed in an aqueous barium acetate solution of a predetermined concentration for 1 hour, pulled up to blow off excess water droplets, and dried at 250 ° C. As shown in FIG. 1, 3.3 g of Pd (supporting portion density: 2.5 g / L) per one obtained catalyst,
t is 0.7 g (supporting part density: 1 g / L), Ba is 40 g
(Loading density: 30 g / L) Further, the length of the Pt carrying portion on the upstream side is 72 mm, which is 50% of the total volume, and Pd and Ba are carried uniformly on the whole carrier. (Example 2) The entire carrier having the same catalyst-supporting layer as in Example 1 was immersed in water, and excess water was blown off. Then, a portion 65 mm from the end face on the downstream side in an aqueous solution of palladium nitrate having a predetermined concentration was applied for 1 hour. It was immersed and pulled up to blow off excess water, and then dried at 250 ° C. Next, the entire palladium-supported carrier is immersed in water, and excess water is blown off. Then, a portion 65 mm from the upstream end face is immersed in an aqueous solution of dinitrodiammine platinum having a predetermined concentration. After blowing off water droplets, drying was performed at 250 ° C.

Further, the entire carrier supporting palladium and platinum was immersed in an aqueous barium acetate solution of a predetermined concentration for 1 hour, pulled up to blow off excess water droplets, and dried at 250 ° C. As shown in FIG. 2, 3.3 g of Pd (supporting portion density: 5 g / L), 0.7 g of Pt (supporting portion density: 1 g / L), and 40 g of Ba per obtained catalyst (see FIG. 2). (Supporting part density: 30 g / L) Further, the length of the Pt carrying portion on the upstream side is 72 mm and 50% of the total volume, and the length of the Pd carrying portion on the downstream side is also 72 mm and 50% of the whole volume.
%. Ba is uniformly supported on the entire carrier. (Comparative Example 1) A catalyst prepared in the same manner as in Example 1 was used in Comparative Example 1 except that the upstream side and the downstream side were reversed. That is, as shown in FIG. 5, the amount of each metal carried was 3.3 g of Pd.
(Supporting part density: 2.5 g / L), Pt 0.7 g (supporting part density: 1 g / L), Ba 40 g (supporting part density: 30)
g / L) as in Example 1, except that the Pt supporting portion is formed 72 mm downstream and is 50% of the whole, and Pt and Ba are uniformly supported on the entire carrier. (Comparative Example 2) The catalyst prepared in the same manner as in Example 2 was used, except that the upstream side and the downstream side were reversed. That is, as shown in FIG. 6, the amount of each metal carried is 3.3 g of Pd.
(Supporting part density: 5 g / L), Pt: 0.7 g (supporting part density: 1 g / L), Ba: 40 g (supporting part density: 30 g / L)
L) is the same as that of the second embodiment, except that the Pt carrying portion is 72 mm on the downstream side and 50% of the whole is formed, and the Pd carrying portion is 72 mm on the upstream side and 50% of the whole. Ba is uniformly supported on the entire carrier. (Comparative Example 3) The entire carrier having the same catalyst-supporting layer as in Example 1 was immersed in an aqueous solution of palladium nitrate having a predetermined concentration for 1 hour.
After pulling up and blowing off excess water, drying was performed at 250 ° C. Next, the entire carrier supporting palladium was immersed in a barium acetate aqueous solution of a predetermined concentration for 1 hour, pulled up to blow off excess water droplets, and then dried at 250 ° C.

As shown in FIG. 7, 3.9 g of Pd (supporting part density: 3 g / L) and Ba
Is supported by 40 g (supporting portion density: 30 g / L).
Pd and Ba are uniformly supported on the entire carrier. (Comparative Example 4) The entire carrier having the same catalyst-supporting layer as in Example 1 was immersed in an aqueous solution of palladium nitrate having a predetermined concentration for 1 hour.
After pulling up and blowing off excess water, drying was performed at 250 ° C. Next, the entire carrier supporting palladium was immersed in an aqueous solution of dinitrodiammineplatinum of a predetermined concentration for 1 hour, pulled up to blow off excess water droplets, and then dried at 250 ° C.
Further, the entire carrier supporting palladium and platinum was immersed in a barium acetate aqueous solution of a predetermined concentration for 1 hour, pulled up to blow off excess water droplets, and then dried at 250 ° C.

As shown in FIG. 8, 3.3 g of Pd (supporting part density: 2.5 g / L) per one obtained catalyst,
Pt is 0.65 g (supporting part density: 0.5 g / L), Ba
Is supported by 40 g (supporting portion density: 30 g / L), and Pd,
Pt and Ba are uniformly supported on the entire carrier. (Evaluation of Purification Performance) Each of the above exhaust gas purifying catalysts was attached to an exhaust system of an engine having a displacement of 2 L, and was operated for 100 hours at an exhaust gas temperature of 800 ° C. and an A / F of 14.6 (stoichiometric). A durability test was performed.

The purification performance of each exhaust gas purifying catalyst after the durability test was evaluated on an engine bench, and the results are shown in Table 1. As an evaluation method, the exhaust gas temperature was maintained at 400 ° C., and the center A / F = 14.6 (stoichiometric) and the center A / F
HC, C at = 14.8 (slightly leaner than stoichiometric)
The purification rates of O and NOx were measured.

[0020]

[Table 1] From Table 1, under the condition of A / F = 14.6 (stoichiometric), both of the examples and the comparative examples show good purification performance and the difference is small. However, A / F = leaner than stoichiometric
Under the condition of 14.8, a remarkable difference appears in the NOx purification performance, and it is clear that the catalyst of the example in which Pt is carried on the upstream side is excellent. (Example 3) As shown in FIG. 3, Pd was 3.3 g (supporting part density: 2.5 g / L), Pt was 0.40 g (supporting part density: 1 g / L) per catalyst, Ba is 40g
(Supporting part density: 30 g / L) The structure is the same as that of Example 1 except that the Pt carrying part on the upstream side is 30% of the total volume. Example 4 As shown in FIG. 4, Pd was 3.3 g (supporting part density: 2.5 g / L), Pt was 0.20 g (supporting part density: 1 g / L) per catalyst. Ba is 40g
(Supporting part density: 30 g / L) The structure is the same as that of Example 1 except that the Pt carrying part on the upstream side is 15% of the total volume. (Example 5) 3.3 g of Pd (supporting part density: 2.5 g / L), 0.14 g of Pt (supporting part density: 1 g / L), and 40 g of Ba (supporting part density) per catalyst : 30g
/ L) carried, and the Pt carrying portion on the upstream side is 10% of the total volume
The configuration is similar to that of the first embodiment except that (Example 6) 3.3 g of Pd (supporting part density: 3.6 g / L), 0.40 g of Pt (supporting part density: 1 g / L), and 40 g of Ba (supporting part density) per catalyst : 30g
/ L) carried, and the upstream Pt carrying part has a total volume of 30%.
%, And the configuration is the same as that of the second embodiment except that the downstream Pd carrying portion is 70% of the total volume. (Example 7) Pd was 3.3 g (supporting part density: 3.0 g / L), Pt was 0.20 g (supporting part density: 1 g / L), and Ba was 40 g (supporting part density) per catalyst. : 30g
/ L) carried, and the upstream Pt carrying portion has a total volume of 15
%, And the configuration is the same as that of the second embodiment, except that the Pd carrying portion on the downstream side is 85% of the total volume. (Example 8) Per catalyst, 3.3 g of Pd (supporting part density: 2.8 g / L), 0.14 g of Pt (supporting part density: 1 g / L), and 40 g of Ba (supporting part density) : 30g
/ L) carried, and the upstream Pt carrying portion has a total volume of 10
%, And the configuration is the same as that of the second embodiment except that the downstream Pd carrying portion is 90% of the total volume. (Evaluation of Purification Performance) The exhaust gas purifying catalyst of each of the above embodiments was attached to an exhaust system of a 2 L engine, and the exhaust gas temperature was 800 ° C., and the A / F was 14.6 (stoichiometric) for 100 hours. A running durability test was performed.

Then, the purifying performance of each exhaust gas purifying catalyst after the durability test was evaluated on an engine bench, and the results are shown in Table 2. As an evaluation method, the exhaust gas temperature was maintained at 400 ° C., and the center A / F = 14.6 (stoichiometric) and the center A / F
HC, C at = 14.8 (slightly leaner than stoichiometric)
The purification rates of O and NOx were measured.

[0022]

[Table 2] As can be seen from the results of Examples 1, 3, 4, and 5, the larger the Pt carrying portion, the slightly leaner the stoichiometric side (A / F =
In 14.8), the NOx purification rate increased, and the NOx purification rate was saturated when the Pt carrying portion was 30% or more. Therefore, considering the cost increase due to the use of Pt,
The optimal amount of the Pt carrier is 15 to 30%. In addition, also from the results of Examples 2, 6, 7, and 8, the Pt carrying portion is 15 to 30.
It turns out that% is optimal.

As the carrier substrate used in the present invention, a monolithic carrier or a pellet carrier having an integral structure is used. Examples of the material include ceramics such as cordierite, mullite, α-alumina, and aluminosilicate, and refractory metals such as an Fe—Cr—Al alloy. As the catalyst support layer, activated alumina is desirable, but in some cases, alumina, silica,
Titania or the like can also be used.

As the alkaline earth metal, barium is particularly desirable, but calcium and strontium can also be used. It is preferable that cerium oxide, zirconium oxide, and lanthanum oxide be contained at the same time, and it is particularly desirable that cerium oxide be contained.

[0025]

That is, according to the exhaust gas purifying catalyst of the present invention, it is slightly leaner than the stoichiometric air (A / F = 14.
The purification performance of NOx in 8) is improved. Further, the alkaline earth metal enhances the NOx purification performance on the rich side, and as a result, the catalyst has a wide window width for A / F.

Therefore, in the feedback control of a general three-way catalyst system, all three components of HC, CO and NOx can exhibit high purification performance.

[Brief description of the drawings]

FIG. 1 is a configuration explanatory view of an exhaust gas purifying catalyst according to a first embodiment of the present invention.

FIG. 2 is a configuration explanatory view of an exhaust gas purifying catalyst according to a second embodiment of the present invention.

FIG. 3 is an explanatory diagram illustrating a configuration of an exhaust gas purifying catalyst according to a third embodiment of the present invention.

FIG. 4 is a configuration explanatory view of an exhaust gas purifying catalyst according to a fourth embodiment of the present invention.

FIG. 5 is an explanatory diagram of a configuration of an exhaust gas purifying catalyst of Comparative Example 1 of the present invention.

FIG. 6 is an explanatory diagram of a configuration of an exhaust gas purifying catalyst of Comparative Example 2 of the present invention.

FIG. 7 is an explanatory diagram of a configuration of an exhaust gas purifying catalyst of Comparative Example 3 of the present invention.

FIG. 8 is an explanatory diagram of a configuration of an exhaust gas purifying catalyst of Comparative Example 4 of the present invention.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yuki Sato 7800 Chihama, Daito-cho, Ogasa-gun, Shizuoka Prefecture Inside (72) Inventor Shinji Matsuura 7800 Chiba, Daito-cho, Ogasa-gun, Shizuoka Cataler (56) References JP-A-5-317652 (JP, A) JP-A-61-78438 (JP, A) JP-A-61-234932 (JP, A) (58) Fields studied (Int .Cl. ⁷ , DB name) B01J 21/00-38/74 B01D 53/86 B01D 53/94 F01N 3/00-3/02 F01N 3/04-3/38 F01N 9/00

Claims (1)

(57) [Claims] 1. Feedback control of a three-way catalyst system
Purifies exhaust gases emitted from the engine
An exhaust gas purifying catalyst, comprising: a carrier substrate; a catalyst supporting layer formed on the surface of the carrier substrate; and a catalytically active component comprising palladium, platinum, and an alkaline earth metal supported on the catalyst supporting layer. And platinum only on the upstream side with respect to the flow direction of the exhaust gas flow.
An exhaust gas purifying catalyst which is carried .